EXCURSION TO THE PYRENEES 43 



In the first case the shales next to the granite, in what is com- 

 monly called the hornstone zone, which contain, as in all contact 

 zones, more or less biotite, have feldspar developed all through 

 them by emanations accompanying the intrusion, being thus con- 

 verted into micaceous feldspathic schists, called in France Lep- 

 tynolites. The usual type of Leptynolite is stated to possess 

 exactly the same structure as the non-feldspathic micaceous 

 schists, but under the microscope it is seen to contain grains of 

 feldspar (orthoclase or plagioclase), which, like the quartz grains 

 in the rock, are allotriomorphic, their form being largely deter- 

 mined by the biotite. The proportion of the feldspar present is 

 extremely variable ; the mineral may be present merely as a few 

 grains here and there in the rock, or the proportion may be greater 

 — the feldspar in some cases being more abundant than the 

 quartz — as in the case of some of the Leptynolite from near 

 Baxouillade. When this is the case, and the rock becomes 

 coarser in grain with an abundance of mica, it passes into a 

 veritable gneiss. 



In the second case the granite is injected at intervals in the 

 form of little veins between the lamellae of the schist, the veins 

 when followed out to their feather edges fading away insensibly 

 into the schist. This is known as "lit par lit" injection, and 

 also gives rise to a gneiss. 



The appearance of the contacts of this class visited on the 

 excursion does not strike the observer as being at all unusual. 

 The explanation which at once suggests itself is that the acid 

 granitic magma which has, by virtue of its acidity, apparently 

 eaten into and dissolved the limestone in certain places and to a 

 limited extent, has not been able to produce any marked effect 

 on the shale. It has forced its way into the shattered strata 

 along the contact, in the usual manner, sending a swarm of little 

 dykes or veins into it in places, and everywhere baking it into a 

 more or less micaceous hornstone-like mass. Fragments of the 

 altered shale, abundant near the contact, less so at some dis- 

 tance, lie scattered about in the granite. These often retain 

 their angular character, but at other times are seen to have been 



