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Show PHYSICAL PROPERTIES OF VOLCANIC ROCKS. 87 crystalline" texture of ordinary lavas, and yet it may not have been erupted or subjected to that mechanical action which is the most conspicuous feature of volcanism. It may have been intruded into a dike, or between strata, and only brought to daylight after the lapse of many geological periods by the agency of denudation. Many of the quartz porphyries and the intrusive or "laccolitic" trachytes of the West, and many basalts or dolerites, are of this character. Are these truly volcanic rocks? Before attempting to answer this inquiry let us advert to the wholly crystalline rocks, such as granite, syenite, diorite, diabase, &c. These are not usually accounted to be volcanic rocks; yet they have been heated and rendered plastic, and they have been intruded into narrow dikes and veins and between strata, though they have never been erupted, so far as we know. Between the intrusive rocks of a wholly crystalline texture and the intrusive rocks of a half-crystalline texture there may be foand a true transition of varieties, and a hard and fast line cannot be drawn between them. Chemically, the two classes are sensibly exact counterparts of each other, and are very nearly so in respect to their constituent minerals. But the failure to find a boundary is no bar to classification, which takes account not only of differences but also of affinities; and hence, while speaking of volcanic and granitoid rocks as distinct classes, we must still keep in mind the reservation that there is a border country between them. Having indicated the characters which belong to all volcanic rocks as a class, and which at the same time serve to distinguish them from other classes, we may next proceed to consider how they differ among themselves, and what affinities exist between the different groups. It may be repeated here that considerations relating to the genesis of rocksâ€"the causes and processes which have made them what they areâ€"should not be directly or primarily taken into the account. We know too little about their genesis, and any attempt to include such considerations would merely lead us to embody what we conjecture rather than what we know, and would almost certainly mislead us. We can take account only of well-known facts, and these are to be found chiefly in those chemical and physical characters which have been extensively studied and compared. These are chiefly as |
