ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 339 



According as a fused silicate cools more or less flowly, the 

 structure of the resulting rock .becomes more or less crystalline. 

 No lava shows on its suface distinct mineralogical characters. 

 Although traces of felspar or augite crystals make their appear- 

 ance sometimes, they are nevertheless rendered unrecognizable 

 by pieces of slag, the cavernous structure of the rock, atmospheric 

 influences, etc. The non-crystalline character of the lava crust is 

 of course attributable to its having been rapidly cooled. The great 

 stream of 1669 from Etna, which is often 60 feet thick is at several 

 places in the neighbourhood of Catania intersected by quarries, in 

 which the structure of its various parts may be studied. It is 

 only at the depth of several feet, that the lava begins to be com- 

 pact and homogeneous. It here consists of a light gray felspa- 

 thic mass in which crystals of black augite and grains of green ol- 

 ivine are disseminated.* Many trachytic lavas of recent produc- 

 tion possess distinctly crystalline characters containing in the 

 compact mass crystals or grains of glassy felspar (sanidine). 



From this sketch of various volcanic processes it would appear, 

 that there are being formed at the present day rocks entirely an- 

 alogous to the basalts and trachytes, which have protruded them- 

 selves almost uninterruptedly through the earth's crust since the 

 commencement of the tertiary period. We observe them solidify- 

 ing from a condition as undoubtedly igneous as that of the slags 

 which flow from our furnaces, and we observe them generally as- 

 suming the form of streams radiating from volcanic craters or as 

 layers on the more horizontal ground around these, which latter 

 form of deposition forcibly reminds the observer of the basaltic lay- 

 ers of much earlier date and non-volcanic origin. Lava is not so 

 frequently observed in the form of veins as are the earlier trac- 

 hytes, nevertheless it is sometimes observed in this form on the 

 sides of craters. The earlier eruptions of trachytic and basaltic 

 rocks seem to have taken place through fissures in the earth's 

 crust, somewhat in the same manner as the older eruptive rocks. 

 The masses thus erupted assumed the form of isolated dome shap- 

 ed hills or wide extended coverings or even of whole stratified sys- 

 tems. In later periods we find these rocks gradually associating 

 themselves with volcanic openings and occurring in the form of 

 lava streams, many of which are even traceable to the craters, 

 which emitted them. Fissures seem to have become more diffi- 

 cult of formation in the crust of the earth and in their place those 



* Sart. Von Waltershausen : Gesteine in Sicilien und Island, p. 100. 



