HEAT — METAMORPHISM. 317 



granite is igneous granite, and if it is forced up opened fissures, it is eruptive 

 granite. 



MetamorpMsm in the above cases is simply crystallization, so far as there 

 is any change ; for chemical change is not needed for the results mentioned. 

 But in many cases it is even simpler than stated ; for in the process the 

 grains of feldspar and quartz may be only enlarged or finished out by surface 

 additions, in a crystalline way, conformably to their crystallographic axes. 

 In a quartz sandstone, the quartz grains, under the process, are made into 

 quartz crystals (Fig. 284) if there is space for it (Sorby); and they may con- 

 tinue growing until the sandstone becomes a compact mass of quartz rock 

 (or quartzyte) showing its original grains only indistinctly. In a similar 

 way, the feldspar grains present in a rock, and any hornblende or pyroxene 

 grains, may be enlarged or finished out. This process would convert a gra- 

 nitic sandstone into granite, making the rock without the heat of fusion 

 or plasticity. In California Cretaceous sandstones, according to Becker, 

 the feldspar crystals made by metamorphic change occupy the positions of 

 previous groups of grains of feldspar ; and the same for pyroxene and 

 hornblende. 



A granitic sandstone having its quartz grains changed to quartz crystals, 

 in a process of metamorphism, becomes thus a qxiartz-porphyry. As quartz 

 crystals are usually formed from siliceous solutions instead of from fusion, 

 the occurrence of such imbedded crystals through the mass of a rock is pre- 

 sumptive evidence against its igneous origin. 



3. Paramorphic Metamorphism. 



When the minerals aragonite and calcite are present together in a 

 limestone (page 69), the first effect of metamorphic action is the conversion 

 of the aragonite into calcite — that is, the making it rhombohedral in 

 cleavage structure, like calcite, its paramorph. Crystallinic metamorphism, 

 also, may go forward simultaneously and make the rock coarsely crystalline. 

 The change of pyroxene crystals to hornblende is a common example of 

 paramorphic change. It has often gone on extensively, changing whole 

 pyroxenic rocks to hornblendic. The inner part of a crystalline grain of 

 pyroxene often has its lines of cleavage crossing at angles of 87°, the angle 

 of pyroxene, when in the outer, the part altered, they are changed to 1241°, 

 the angle of hornblende._ The altered pyroxene was named uralite (from 

 the Urals) by G. Rose (1830), and the change is hence called uralitization. 

 Many Archsean crystalline rocks now hornblendic have been proved, by such 

 evidence, to have been originally pyroxenic ; and so it is with many other 

 rocks, including some of igneous origin. Even the pyroxene of dikes of 

 doleryte has been found changed to hornblende. 



This change in a rock of the basalt type (the doleryte of Land's End, Cornwall) was 

 first observed by AUport (1876); in augite-syenyte of New Hampsliire, by S. W. Hawes 

 (1878); in Wisconsin Archsean rocks, by Irving and Van Hise (1883). The mineral hy- 



