POSITION OF THE PORPHYEIES IN CLASSIFICATION. 



97 



Texture, then, if the foregoing views be true, is associated with the 

 genesis of rocks and is determined by the conditions under which the rocks 

 have soHdified. Although it may seem to be a trivial character, in reality 

 it is a very important one, since it is an index of conditions and occur- 

 rences of vital importance to the genesis of the rocks and their geological 

 relations. For it is of the highest geological importance to know whether 

 certain rocks have been erupted or have been formed in situ; whether they 

 . are indigenous or exotic. The indications given by texture may be uncer- 

 tain at times, and occasionally even misleading; but on the whole, so far 

 as they are now understood, they may be relied upon. The differences of 

 texture have heretofore been employed chiefly to distinguish the eruptive 

 from the non-eruptive igneous rocks. The wholly crystalline are non- 

 eruptive ; the partially crystalline are eruptive. But, although the wholly 

 crystalline rocks are not commonly found in the form of lava sheets or 

 coulees, they are occasionally found in the form of intrusions, and so, also, 

 are the partially crystalline rocks. The intrusive condition is, therefore, a 

 kind of intermediate stage between the eruptive and non-eruptive condi- 

 tion, representing an abortive attempt at eruption, sometimes resulting in a 

 slight displacement of the magma, sometimes almost accomplishing an out- 

 pour. In very many cases — probably in many more than we are now jus- 

 tified in affirming — this qualified eruption is associated with a texture which 

 seems to be characteristic of it, the porphyritic texture. 



A satisfactory definition of "porphyry" is almost impossible to find. 

 The most general conception is that it applies to a rock consisting of crys- 

 tals, usually feldspar and quartz, imbedded in an "unindividualized" paste 

 or base; but forty-nine-fiftieths of all intrusive and eruptive rocks come 

 fully within such a definition. Except an insignificant quantity of obsid- 

 ians and aphanitic rocks, all volcanics are decidedly porphyritic. And 

 3'et lithologists employ the term to designate a group of rocks different 

 from volcanics, not only in their geological relations, but in their appear- 

 ance as dependent upon texture. There are certainly some rocks which 

 we do not hesitate to call porphyry, and regard them as being quite distinct 

 from the common lavas; the distinction, moreover, being a textural and 

 not a chemical one. As nearly as we can reach a description of the spe- 

 7 H p 



