part 4] PETROLOGY OF THE ARNAGE DISTRICT. 459 



in the norite, and forms large patchy aggregates of grains ; 

 commoner, too, are the hiotite and hornblende. 



The second variety of this type is fine-grained, and is found, 

 associated with much-contaminated rocks, in the Nether Mill knob, 

 at Mill of Elrick, or in the wood east of Waulkmill of Savoch. 

 In slice the structure is granular, and the rock approaches a 

 beerbachite. Not only is the structure different from that of 

 the norite, but the pyroxene is now mainly monoclinic. The rock 

 consists of subhedral prisms of acid labradorite, granular pyroxenes, 

 brown-green hornblende, biotite, quartz, and magnetite. Like their 

 more contaminated associates, these quartz-gabbros have often a 

 fairly well-defined fluxional structure. 



There also occur, at Towie Wood and elsewhere, a few small 

 patches of contaminated rock which are like the Arnage Type 

 to be next described, but show a small amount of hypersthene, 

 in addition to much quartz, acid labradorite, biotite, often horn- 

 blende, and rarely cordierite. These patches are closely associated 

 with contaminated rocks of the Arnage Tj'pe, into which they pass 

 gradually by loss of hypersthene and increase of cordierite. 



In all these gabbro-types the plagioclase is richer in the albite- 

 molecule than is that of the norite. 



(B) Quartz-Biotite-Felspar-Cordierite-RockSj or 

 the Arnage Type. 



Mode of occurrence. — As already stated, this type of con- 

 taminated rock is found mainly north of a line drawn from Mill of 

 Kinharrachie to the southern end of the Little Arnage kernel, and 

 thence to Cookston ; it is the most widespread of the contaminated 

 types, and, in the whole Arnage Mass, occurs over an area of about 

 13 square miles. In the field the rocks of this type are extremely 

 variable in structure, and in the relative proportions of their com- 

 ponent minerals ; but all varieties agree in showing partly digested 

 sedimentary xenoliths, which are often sufficiently abundant to 

 impart to weathered surfaces of the rock a conglomeratic aspect, 

 These xenoliths are described on pp. 473-78 ; here only the con- 

 taminated matrix is dealt with. Anywhere outside the kernels 

 this type and its xenoliths can be studied, but especially beautiful 

 phenomena can be seen at the following localities : — 



(1) Towie Wood, at the southern end of the Inkhorn 

 kernel, a mile and a quarter north of Arnage Station. — 

 In the ground immediately east of the railway, and especially in 

 a rocky knoll rising out of a peaty fiat there, the contaminated 

 rocks enclose abundant xenoliths of hornfelsed quartzite and blue 

 argillaceous hornfels. The bedding of the larger of these hornfels- 

 blocks has a constant dip over the area exposed, pointing to root- 

 contamination ; they are described in detail on pp. 474-78. Often 

 the contaminated rock forms a 1 it-par-lit complex with the 

 argillaceous hornfels, and all stages of mixture of hornfels and 

 magmatic rock can be observed : the first stage consists of the 



