1866.] J. GEIEIE CAREICK, AYRSHIRE. 517 



varies, of course, 'with the texture of the rock ; it is usually rough 

 and irregular, like that of many traps, but sometimes conchoidal. 



Amygddloidal Cavities. — The amygdaloidal cavities are very 

 abundant, but are often partially aggregated in areas, leaving the 

 surrounding portions of the matrix less profusely pitted. They 

 invariably assume a globular shape, except where two or more have 

 become confluent, when the cavity resulting from their union is 

 usually of irregular form. They are of all sizes, from mere points 

 up to globules as large as a sweet pea ; but larger cavities are not un- 

 common. Perhaps the most abundant sizes are those which may be 

 compared to turnip and mustard seeds. Carbonate of lime is the 

 mineral most usually met with in these cavities ; but zeolites and 

 decomposing magnesian minerals are abundant in places ; hard 

 white felspar is also occasionally met with. 



The most continuous exposure of the rocks in question occurs 

 along the coast, between Knockgowan Hill and the river Stinchar. 

 Throughout this section the bedding is often very distinct, the 

 strike corresponding with that of the unaltered greywackes of the 

 adjoining districts, viz. N.E. and S.W. But frequently all trace of 

 bedding is lost, even when the rock-masses are viewed from a little 

 distance. Here and there occur bands of a hard and highly altered 

 red greywacke mudstone passing into Lydian stone ; and occasionally 

 we come upon portions of fine pebbly and brecciated greywacke, 

 resembling at a first glance the comminuted paste of a volcanic ash. 

 Courses of greywacke, distinctly granular, alternating with thinner 

 bands and shales, also make their appearance in some parts of the 

 section. AH these regularly bedded and undoubtedly aqueous rocks 

 are interstratified with, and, when their strilie is prolonged, seem to 

 pass into, felspathic amygdaloid and finely crystalline felstone. 



It has been stated that a first view of the amygdaloid would lead 

 one to infer an igneous origin for that rock : to one, however, who 

 had previously explored the greywackes of the surrounding country, 

 and familiarized himself with their texture and mode of weathering, 

 this apparently igneous aspect is not always so striking. In many 

 places the greywackes of the non-metamorphic regions are exceed- 

 ingly hard, and sometimes approach to crystalline in texture. More- 

 over the weathered crust of the amygdaloid, like that of the grey- 

 wackes, is seldom more than a line or so in thickness ; and the rock 

 when broken shows a very general absence of that dull earthy tex- 

 ture which among Scottish traps usually attends an amygdaloidal 

 structure. 



From the distinctly bedded nature of the amygdaloids, and their 

 alternation with other rocks to be described in the sequel, these 

 masses, if of igneous origin, are manifestly not intrusive. On the 

 other hand, if we beheve them to have been poured out at the earth's 

 surface, we shall look in vain for those flattened amygdaloidal or 

 vesicular cavities so characteristic of true amygdaloidal trap. The 

 shape of the vesicles, as already mentioned, is invariably spherical. 

 If this appearance were confined to certain limited areas, or only 

 showed itself irregularly here and there, we could not consider it 



