This one will be a little shorter--getting tired and I still have stuff to do tonight.
Mountain Top Mystery by Gertrude Chandler Warner
The Alden's go for a day trip to a nearby mountain and end up stranded overnight when a rock slide breaks the trail down. Said rock slide opens up a cave and a treasure is discovered.
This story is less a mystery than some of her other books. The children are a little older, though Violet is inevitably fragile--which I find a little annoying. With all the running about these kids do, you'd think she'd have developed some health, if not some gumption but she's mild and sweet--always.
Warner speaks of an Indian tribe that used to live in the forest near the mountain, with the two remaining survivors coming together and finding a treasure of French silver as the "mystery" portion of the book. It was a little unbelievable that a young boy of 12 could hitchhike and disappear in and out of camps but considering how Warner started the series, perhaps it's not entirely unreasonable for her imagination.
Has anyone else ever noticed that the Alden children's parents are NEVER mentioned, they never seem to grieve for them, and it's amazingly unclear what they were doing before the first story? These are well educated, nicely brought up children---even assuming they were estranged from their grandfather because of something between the parents and him, why were there no friends of their parents to look after them? Where was their mother's family? How do four children wander off into the night with only one change of clothing and no one notices?